The present invention relates to a nuclear magnetic resonance (NMR) magnetometer probe. It is used in the precise measurement of magnetic fields, particularly the geomagnetic field, whose value is 5.10.sup.-5 Tesla or 50,000 gammas, the gamma being a particularly suitable unit for use in the envisaged application (1 gamma=10.sup.-9 Tesla).
The magnetometer in which the invention is used is of the spin coupling oscillator type without a forbidden axis. Such an apparatus is known and is more particularly described in French patent application Nos. 1 447 226 and 2 098 624. There is consequently no need to provide a detailed description of such an apparatus here. It is merely pointed out that this apparatus comprises (according to FIG. 1) at least two liquid samples contained in bottles (four in the illustrated variant, i. e. 2. 4. 6. 8), said bottles being placed in a very high frequency resonant cavity. The latter is constituted by a central conductor 10 and an outer conductive wall 12, the central conductor and wall being connected to a coaxial cable 14 connected to a not shown VHF generator. The probe also comprises windings 16, 18 for sampling and reinjecting a signal at the Larmor frequency defined on the one hand by the magnetic field in which the probe is located and on the other by the natural gyromagnetic ratio of the samples used.
In this connection it is pointed out that the term "sample" designates a solvent mass having atomic nuclei with a magnetic moment and angular momentum of a non-zero nature, i.e. with a well defined gyromagnetic ration. This solvent contains in solution, a paramagnetic substance having at least one saturatable electronic line of resonance. The latter is excited by the very high frequency field established in the resonant cavity. As the resonant frequency of this cavity cannot be defined with sufficient accuracy, use is made of a tuneable member, normally of a capacitive nature, which makes it possible to adjust the resonant frequency in the laboratory.
In the aforementioned prior art and particularly FR-A No.2 098 624, the regulatable capacitive element has the shape shown in the right-hand part of FIG. 1. It is an e.g. quartz disk 15 having a central hole and which is coated on its outer face with silver-plated sectors 17 connected to the outer conductive wall 12. These sectors do not extend up to the central hole of the disk. On the inner face, the disk is coated with silver sectors of limited thickness 19, which face sectors 17. Sectors 19 are interconnected by a metal deposit 21, which covers the central hole and at the same time constitutes a connection to the central conductor 10.
The thus formed assembly acts as a capacitor, sectors 17 and 19 constituting the coatings and the quartz disk 15 the dielectric. This capacitor is inserted between the central conductor 10 and the outer wall 12 of the cavity. The adjustment of this capacitor consequently permits the regulation of the resonant frequency of the cavity, for which purpose the operator scratches to a greater or lesser extent the silver plating 17 covering disk 15.
Although satisfactory in certain respects, this regulating means suffers from disadvantages. Firstly, it is bound by the shape of the bottles used which, as can be seen in FIG. 1, are cylindrical. However, this is not always the case, because the shapes can vary widely and can e.g. be hemispherical. Moreover, the apparatus is essentially asymmetrical, because it is located at one end of the probe. It is also relatively complex and expensive, because it requires a quartz disk covered with silver-plated sectors. Finally and as will be made more apparent hereinafter, the inevitable presence of a bubble in the end bottle disturbes the regulation when the probe is in the vertical position, particularly in the case of hemispherical bottles.